But it's a bit like trying to find a needle in a haystack, when you have no clue where the haystack is.
"In particular, we can offer no argument that an advanced, intelligent civilization necessarily produces narrow-band radio emission, either intentional or otherwise," the astronomers caution in a paper submitted to the arXiv pre-print service today. "Thus we are probing only a potential subset of such civilizations, where the size of the subset is difficult to estimate.
"The search for extraterrestrial intelligence is still in its infancy, and there is much parameter space left to explore."
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86 stars were selected for this directed search, and over a period of three months in 2011 (February to April), the Green Bank antennae detected 52 candidate narrow-band radio signals (less than 5 Hz) between 1-2 GHz. There are no known natural mechanisms for generating radio signals at such narrow frequencies. "Emission no more than a few Hz in spectral width is, as far as we know, an unmistakable indicator of engineering by an intelligent civilization," they added.